Here's a list I've been working on. Criteria is it must be half-hour format for a sitcom AND I bend the rules a little on when it first came out vs when seinfeld ended, along with best season. I also think about a show when it's at its peak:
1. Arrested Development (2003), S2 or S3
The show has been pretty evenly excellent. The levels of neuroses that the Bluth family is so complex and hillarious at every turn. Seinfeld was about nothing but it had a twisted way from getting to start to finish. Arrested Development has those same kind of disparate story lines that connect, but it has to work through it's own internal logic much more since it has such a complex web of characters (including one of the best arrays of guest stars ever produced over a three-year run). It also has heart (or a sort of satirical version of it). The best thing, however, is that repeat viewings never ever disappoint.
2. The Office (2005), S2
At its peak, The Office was the most relatable show on television and the humor of tension and awkwardness was so masterful. How many shows have you so engrossed in a moment of tension that you're yelling at the TV. The show declined a little as Jim and Pam's romance has become pedestrian, but the Office has wisely expanded its ensemble so that it has characters on the back burner it can throw at the fire to spice things up.
3. 30 Rock(2006) S2
On 30 Rock, Tina Fey writes comedy with such dexterity and precision, that scripts to her show should be used as textbooks to aspiring TV writers because she layers her scripts every kind of comedy imaginable: screwball comedy, anarchic comedy, satire, parody, character-based comedy, rapid-fire-blink-and-you-miss-it dialogue, straight-man-funny-man comedy, self-conscious comedy, etc. The key to the show is Tina's overworked head writer Liz Lemmon being the only basis for normality in this wonderful universe she's created that's ever so slightly off-center.
4. 3rd Rock from the Sun(1997) S1 (technically, this show overlapped with Seinfeld by a season or two).
3rd Rock from the Sun is one of those fish-out-of-water premises like The Beverly Hillbillies that when handled well never gets old. The Solomon family is a family of four in Middle America but they're also a team of aliens sent to Earth to learn more about it. Actors as talented as John Lithgow, Joseph Gordon-Leavitt, and Jane Curtain sold the material too.
5. Newsradio (1995): S2 and S3
The best of the Manhattan-based nondescript workplace comedies that came out in the late 90's (Working, Caroline in the City, the Naked Truth, etc. are more examples). Phil Hartman, Andy Dick, Joe Rogan and Vikki Lewis created malleable comic characters that could be funny in any situation you threw them in. Dave Foley and Maura Tierny had a screwball comedy for the ages (not to mention the hilarious sexual tension that arose out of that single sort-of-date between Joe Rogan and Khandi Alexander's characters). Throw in Stephen Root as the least sensible boss ( yet somehow the most successful) and you've got quite a show.
6. Futurama(1999) S2
I think it's safe to say that Matt Greoning's futuristic universe is one I'd like to live in. It looks so neat and it's apparent that the Futurama creators had just as much fun making that universe as you do watching it. Part sci-fi, part satire (a lot of references to 20th century Earth), part comedy, Futurama is a tremendous amount of fun. The characters (particularly Bender) are just a couple of tics away from completely narcisstic and dislikeable but the show has enough heart to overcome these gaps. The episode plots are always highly inventive as they play with the Rosewell incident (what sci-fi show doesn't?), alternate universes, university life in the 30th century, the lost city of Atlanta and more.
7. Spin City(1996) S1
Spin City was definitely not a very progressive sitcom but it made for a great ensemble comedy with fast-slinging dialogue and stinging humor coming from all directions. Michael J. Fox was impeccably cast to his role and made a great comic character and supporting roles by Alan Ruck, Jennifer Esposito, Connie Britton, Michael Boatman, Barry Bostwick, Richard Kind, Alexander Chaplin provided a colorful palette for humor. The show went downhill in later seasons when Fox started being eclipsed by Heather Locklear and eventually replaced by Charlie Sheen. Another source for the decline was when the characters started becoming less believable as city hall employees since they became increasingly goofy and caricatured.
8. Family Guy (1999) S6
When I start downloading an episode on itunes, I have doubts and ask myself, "Is this going to be worth it?" because I continually forget how great this show is. The sheer depth with which Seth MacFarlane and his staff can rip across pop culture is part of the fun, but it wouldn't be the same (for proof of this, just watch Seth MacFarlane's comedy clavacade) without the situational humor of the characters: Especially Brian and Stewie.
9. Just Shoot Me(1999) S3
Another nondescript workplace comedy that often gets overlooked. If I had to pinpoint one thing that makes the show so great, it's how well the five characters complement each other. Each of them has strengths and needs that the other wants. Maya is possessed with intelligence and the ability to think for herself, but she wishes she had the beauty of Nina and the interpersonal skills and love of her father. Jack, highly successful as a businessman and well-liked person among friends, has been successful at everything in life except being a good father. Nina, Elliot and Dennis are all childlike and helpless in different ways and need Jack as a father figure to guide them. What's really great about all of this is that this layer of wants and needs that drives the group together is balanced with a surface-level atmosphere of bickering, put-downs, office pranks, and even people stepping over each other to get ahead. But at the end of the day, it's about how what's most important is the sense of what each person brings to the people around him and the sense of family that's formed. Just Shoot Me is for every straight-A student who wishes she were prettier, every class clown who wishes he would have the respect of the valedictorian, every successful businessman who wished they could connect to their family, every nerd who wishes they could be friends with the prettiest girl in the room, and every overgrown child who wishes their boss could be more like the dad they never had.
10. It's Always Sunny in Philadelphia (2005) S4
The last season they really started picking the show up. First off, the show's dialogue is truly one of a kind: The way in which they all start talking over each other and gradually turning practically any civilized situation into complete chaos must be so difficult to write for. I love how these guys are so narcissistic yet so loveable at the same time. I think what's tacitly acknowledged is that the characters have some good qualities: They are, in their own way, more loyal to each other than anyone else is to them and they are unusually ambitious in their quest to better themselves. Of course they wind up screwing each other over by the end of the episode and they do things for the basest of instincts, but don't you want to secretly hang out with them? They are walking and talking affronts to political correctness in society. They're pretty much not afraid to broach anything. In particular, they have a lot of fun mocking the concept of the father figure for some reason.
11. American Dad(2005) S4
The show started out horribly but as it progressed, the writing shifted from political satire to a unique take on the American family sitcom where every cliche is driven to the extreme. Seth MacFarlane, showrunner of Family Guy, took this on as his sophomore project, and shows that he can make you laugh without using cutaways. Of course, like Family Guy, they add in two completely ridiculous additions to the ensemble to shake things up: Klauss, the talking fish, and Roger, the sexually-ambiguous self-involved alien.
12. Aliens in America (2007) S1
The show only lasted one season, unfortunately, but it was truly one of the most astute and painfully self-aware takes on the high school comedy genre I've ever seen. With the addition of Raja, the Pakistani exchange student, the show also doubled as a satire on "Middle American" ignorance.
13. Extras (2005) S1
This was a show like the British counterpart of "The Office" that ran way too short. The concept of fame and how far people are willing to go to obtain it was the source of this comedy about a guy who goes from film to film as an extra in hope of finally being able to break through to an acting career. The appearance of guest stars and the sheer unpredictability of what they would do added to the fun, but the show stood on its own merits as well.
14. 8 Simple Rules (2002) S3
As someone who grew up on family sitcoms, I'm a sucker for the traditional three-camera format and "8 Simple Rules" has been the best family sitcom of the last decade. The show started off particularly generic but when James Garner and David Spade joined the cast, things really started to gel and Katie Segal was already a pro. Again, it wasn't groundbreaking but who needed to expect anything better?
15 (tie). My Name is Earl (2005) S2
Part of the secret of comedy is that its about making us feel good inside, and at times My Name is Earl isn't a sitcom but a parable. At the same time, there's enough comedy in there to mine a sitcom out of due mostly to the solidness of the cast. Jamie Pressley, the bitchy ex-wife, is definitely the high point of the show and she won a deserved Emmy for the show. A lot of the show is making fun of trailer trash or lower income people and the comedy is kind of "look at how dumb these people are," but it's somewhat redeeming that by writing the characters played by minorities, Darnell and Catalina, as the smartest characters.
Big Bang Theory (2007) S2
Like a number of shows, it took a few episodes for the characters to develop and grow on us, but once that happened, Jim Parsons and Jimmy Galecki become pretty addictive as theoretical physicists with varying degrees of social ineptness. Kaley Cuckoo is charming as the girl next door (even though, honestly, how many girls that good-looking would hang around those guys in real life?). One problem is that for all the growth Leonard (Galecki) and Sheldon (Parsons) have made, the two side characters, Raj and Wolfowitz, have stayed pretty one-dimensional. Another one of those shows to watch just to see an example of how good stylized dialogue is written.